Aluminum oxide coated cemented carbide product

ABSTRACT

A high-strength, coated cemented carbide product comprising a cemented carbide substrate and a fully dense alpha aluminum oxide coating on the substrate. The coating has a thickness of from 1-20 microns and is firmly and adherently bonded to the cemented carbide substrate through a thin intermediate nonmetallic layer .[.of an iron group metal aluminate.].. The coated product combines a wear resistance substantially as high as aluminum oxide cutting materials and a transverse rupture strength of at least 150,000 p.s.i. The coated product is prepared by passing water vapor, hydrogen gas and an aluminum halide over the substrate at a temperature of from 900°-1250° C., the ratio of water vapor to hydrogen gas being between about 0.025 and 2.0.

BACKGROUND OF THE INVENTION

This invention relates to a high-strength, coated cemented carbideproduct and to a process for its preparation.

Cemented carbides are well known for their unique combination ofhardness, strength and wear resistance and are accordingly extensivelyused for such industrial applications as cutting tools, drawing dies andwear parts. It is known that the wear resistance of cemented carbidesmay be enhanced by the application of a thin coating of a highlywear-resistant material, such as, for example, titanium carbide, andsuch coated cemented carbides are finding increasing commercial utilityfor certain cutting tool and machining applications. However, theincreased wear resistance of such coated products has been at thesacrifice of the strength of the substrate which is substantiallyreduced after coating.

Because of its high hardness, wear resistance and low reactivity with awide variety of metals, aluminum oxide has excellent potential as a toolmaterial, and this potential has to some extent been realized with avariety of aluminum oxide cutting materials that are commerciallyavailable. The principal drawback to the more widespread use of aluminumoxide tools in their low strength which rarely exceeds 100,000 p.s.i.,using the standard transverse rupture or bend test. This compares with astrength of from 200,000 to 300,000, or even more, for cemented carbidecutting tools. The low strength of aluminum oxide tools limits their useto cutting applications where the tool is not highly stressed, such asin finishing cuts. The low strength of aluminum oxide also precludes theuse of such materials in certain types of insert shapes which encounterhigh stresses when locked in a toolholder.

It is an object of this invention to provide a hard, wear-resistantmaterial which combines the extremely high wear resistance of aluminumoxide with the relatively high strength and hardness of cementedcarbide.

It is an additional object of this invention to improve the wearresistance of cemented carbides without substantially reducing theirstrength. It is still an additional object of this invention to providea process for producing a firmly adherent, nonporous, dense coating ofaluminum oxide on a cemented carbide substrate.

SUMMARY OF THE INVENTION

The foregoing and other objects of this invention are achieved by thevapor deposition under carefully controlled conditions of an alphaaluminum oxide coating of from 1-20 microns thickness on a cementedcarbide substrate. The product contains a cemented carbide substrate anda fully dense alpha aluminum oxide coating firmly and adherently bondedto the substrate. In addition, there is present a very thin,intermediate nonmetallic layer .[.of cobalt-, iron-, or nickelaluminate.]., which acts to metallurgically bond the coating to thesubstrate. The coated product has a wear resistance substantiallyequivalent to aluminum oxide base cutting materials and a transverserupture strength of at least 150,000, in most cases greater than 200,000pounds/sq. inch. At very high cutting speeds, greater than about 1,500surface ft./minute in some applications, possibly higher in others, thehigher heat resistance of solid aluminum oxide may result in higher wearresistance. But in all cutting tests other than those above theselevels, the wear resistance of the present coated products has proven tobe substantially as high as aluminum oxide cutting materials.

While the broad range of coating thicknesses useful in the invention isfrom 1-20 microns, most coating thicknesses are preferably less than 15microns. As will be shown in more detail below, certain applicationsrequire even narrower ranges within these limits, e.g. 1-3 microns hasproven optimum for machining high temperature alloys and for millingapplications; 6-12 microns has proven optimum for steel machining.

The process of the invention comprises passing an aluminum halide, watervapor and hydrogen gas over the carbide substrate at a temperature offrom 900°-1250° C., the ratio of water vapor to the hydrogen gas beingmaintained between about 0.025 and 2.0, and preferably between 0.05 and0.20.

There have previously been references in the literature of attempts orsuggestions to coat a variety of substrates with aluminum oxide.However, insofar as is known, the coating of a cemented carbidesubstrate with aluminum oxide to produce a fully dense and adherentcoating has never previously been disclosed. Nor has the unusualcombination of properties exhibited by the present products beenpreviously attainable in either coated or uncoated cutting toolmaterials. The products of the invention are remarkable in severalrespects. Their strength as compared with comparable known coatedcemented carbide materials is considerably higher and their cuttingperformance is superior in terms of tool life at intermediate and highercutting speeds. The basis for the foregoing statements will becomeapparent from the discussion and test results set forth below.

The term cemented carbide as used herein means one or more transitionalcarbides of a metal of Groups IVb, Vb, and VIb of the Periodic Tablecemented or bonded by one or more matrix metals selected from the groupiron, nickel and cobalt. A typical cemented carbide contains WC in acobalt matrix or TiC in a nickel matrix.

Because of the demanding requirements normally placed upon a cementedcarbide cutting material, the properties of any coating, the manner inwhich it is bonded to the substrate and its effect on substrate strengthare extremely critical. The coating layer must have high integrity interms of density and smoothness--porosity or nonuniformity cannot betolerated. The coating must also be firmly and adherently bonded to thecemented carbide substrate to prevent spalling or separation in use. Inaddition, the coating must not reduce the strength of the cementedcarbide substrate significantly. The products of the present inventionhave been extensively tested and have been found to satisfy all of theforegoing requirements. The coatings are uniform and fully dense, theyare firmly bonded to the substrate and the coated composite retains ahigh proportion of its strength, usually greater than 85% of thetransverse rupture strength of the uncoated substrate. The achievementof these characteristics in the coated product is believed to be quiteunexpected, particularly in view of the substantial strength reductionsknown to result from the addition of wear-resistant coatings to cementedcarbide substrates. The coated materials of the invention also produce asurface finish in machining operations which appears to be fullyequivalent in quality to solid aluminum oxide cutting materials, thelatter being known to produce the best surface finishes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The outstanding properties of the aluminum oxide-coated product of theinvention depend upon careful control of the process parameters. Theprocess involves the use of a gaseous mixture of hydrogen, water, and analuminum halide such as aluminum trichloride. Carbon monoxide and carbondioxide may be optionally added. The primary overall deposition reactionis:

    3H.sub.2 O+2AlCl.sub.3 →Al.sub.2 O.sub.3 +6HCl

The most important ingredients in the gaseous reaction mixture aretherefore water vapor and aluminum chloride vapor. However, the aluminumchloride vapor can be formed in several ways during the depositionreaction, as for example by heating solid AlCl₃ powder or by passingchlorine gas over aluminum metal. The water vapor is most convenientlyformed by reacting hydrogen with carbon dioxide in the depositionchamber to form carbon monoxide and water vapor by the water gasreaction:

    H.sub.2 +CO.sub.2 ⃡CO+H.sub.2 O

The amount of water vapor formed in this manner depends upon thetemperature and the initial concentrations of hydrogen, carbon dioxide,carbon monoxide and water vapor in the input gas stream. In order toform a good quality coating of desirable thickness in the temperaturerange of 900°-1250° C., the ratio of water to hydrogen gases present,after the water gas reaction, should be between about 0.025 and 2.0.

Hydrogen has been found to be necessary in the vapor deposition processto obtain a dense, adherent coating. Hydrogen appears to insureoxidation of the aluminum at the carbide surface. Oxidation in thereaction zone above the carbide substrate creates a condition known asdusting--which must be avoided. The absence of hydrogen creates a porouscoating which is not fully dense. Thus the three necessary ingredientsof the process are aluminum halide vapor, water vapor and hydrogen. Inits preferred form, the process includes the use of aluminum chloridevapor, hydrogen and carbon dioxide, the latter reacting with H₂ to formwater vapor.

The amount of water vapor present, after the reaction of known inputconcentrations of H₂ and CO₂ and CO and H₂ O if used, can be calculatedusing the following equation: ##EQU1## where

a=1-K; K=the equilibrium constant for the water gas reaction;

b=(CO)_(i) -(H₂ O)_(i) +K((H₂)_(i) +(CO₂)_(i) +2(H₂ O)_(i)); and

.[.c=K((H₂)_(i) (CO₂)_(i) +(H₂)_(i) (H₂ O)_(i) +(H₂ O)_(i) (CO₂)_(i)+(H₂ O)_(i) ²).]. .Iadd.c=--K((H₂)_(i) (CO₂)_(i) +(H₂)_(i) (H₂ O)_(i)+(H₂ O_(i) (CO₂)_(i) +(H₂ O)_(i) ²) .Iaddend.

The parentheses denote the concentration of the gaseous species enclosedwithin in terms of partial pressure, and the subscripts f and i denotethe final or equilibrium concentrations and the initial or inputconcentrations, respectively. The amount of H₂ present, and thus the H₂O/H₂ ratio, can then be determined from the relationship:

    (H.sub.2).sub.f =(H.sub.2).sub.i +(H.sub.2 O).sub.i -(H.sub.2 O).sub.f

A series of coated products were prepared in accordance with theinvention by passing aluminum chloride vapor, hydrogen and carbondioxide over cemented carbide inserts. The examples were prepared atvarious input gas compositions and at various final H₂ O/H₂concentrations. In all cases, deposition was at 1050° C. and a 45-minutedeposition cycle was used with 2-3 grams of aluminum chloride and analuminum chloride generator temperature of about 200° C. The use of moreAlCl₃ shifts the desired H₂ O/H₂ ratio to a higher value and vice versa.The coatings were deposited on a cemented carbide substrate having thefollowing composition in percent by weight: WC-72, Co-8.5, TiC-8,TaC-11.5. Table I below shows the effect of gas composition on coatingthickness. When coating with both higher and lower ratios of H₂ O/H₂(i.e. outside range of about 0.025 to 2.0), it wasn't possible to get acoating of sufficient thickness, i.e., >1 micron. Coating quality wasgood for all examples having more than 1 micron thickness coating.Coating quality was judged to be good if the coating could withstand anadherency test consisting of sliding the coated insert under a diamondbrale indentor of the same type used for the Rockwell hardness testusing a load of 2 kilograms on the diamond. If the coating resistedspalling or crumbling during this test, it was judged to have goodquality. If it did not, it was judged to have poor quality.

                                      TABLE I                                     __________________________________________________________________________                            Water gas reaction                                      Example                                                                           (H.sub.2)(CO.sub.2)(CO)(H.sub.2 O)Input gas partial                                              (H.sub.2)+(H.sub.2 O)+partial pressuresequilibriu                            m                                                                                        ##STR1##                                                                            (microns)thicknessCoating            __________________________________________________________________________    1    .978                                                                             .022  .000  .000                                                                              .956 .0217                                                                              .023   <1                                   2    .960                                                                             .040  .000  .000                                                                              .921 .0391                                                                              .043    21/2                                3    .850                                                                             .150  .000  .000                                                                              .713 .137 .192    6                                   4    .750                                                                             .100  .150  .000                                                                              .665 .085 .127    6                                   5    .050                                                                             .Iadd..100.Iaddend..[..850.].                                                       .Iadd..850.Iaddend..[..100.].                                                       .00 .042 .0085                                                                              .205    3                                   6    .600                                                                             .400  .000  .000                                                                              .323 .277 .857    3                                   7    .450                                                                             .250  .300  .000                                                                              .307 .143 .466    9                                   8    .400                                                                             .600  .000  .000                                                                              .123 .277 2.26   <1                                   9    .100                                                                             .700  .200  .000                                                                              .019 .081 4.26   <1                                   10   .975                                                                             .000  .000  .025                                                                              .975 .025 .026    1                                   __________________________________________________________________________

The nature of the coating obtained was determined by using X-raydiffraction analysis and optical microscopy. X-ray analyses showed thecoating to be alpha Al₂ O₃. At the higher deposition temperatures(greater than 1150° C.), significant amounts of the compound .[.W₃ CO₃C.]. .Iadd.W₃ Co₃ C .Iaddend.began to form due to reaction of thesubstrate carbide with the coating atmosphere. Optical microscopyrevealed a gray, translucent coating of Al₂ O₃ that was fully dense andwell bonded to the substrate in those examples in which the coatingquality was found to be good. A very thin (less than 1 micron) layer ofanother nonmetallic compound.[., cobalt aluminate (CoAl₂ O₄),.]..Iadd.formed by oxidation of the titanium containing carbide phase ofthe substrate (a titanium containing oxide of the rutile-type.Iaddend.was present between the Al₂ O₃ layer and the cemented carbidesubstrate. The presence of this thin layer is necessary to achieve theproper bond strength between the coating and the substrate, that is, abond strength sufficient to pass the adherency test set forth above. Inthose cases in which no observable intermediate nonmetallic layer waspresent, the coated inserts did not pass the above described adherencytest.

For this reason, .[.a cobalt (iron, or nickel) aluminate.]. intermediatelayer is believed necessary to a good quality coating.

The preferred temperature range for deposition of the coating is 900° C.to 1250° C. At lower temperatures, the deposition rate becomes very lowand the coating is poorly bonded to the substrate. At highertemperatures, excessive reaction occurs between the coating atmosphereand the cemented carbide substrate, weakening the bond between thecoating and the substrate and lowering the strength of the overallcomposite body.

The strength of the Al₂ O₃ coated cemented carbide composite wasmeasured (as were all strength measurements disclosed herein), using aslightly modified standard transverse rupture test (ASTM No.B4066--63T), that included three roll loading and a span-to-thicknessratio of 3.5 to 1. Using a deposition temperature of 1050° C. and acemented carbide substrate of the nature set forth in the first tenexamples in Table I above, the average strength of bars having coatingthicknesses of from 5-7 microns (the preferred thickness for thissubstrate in terms of wear resistance) was 241,000 p.s.i. Thisrepresents only a slight reduction (11%) from the 270,000 strength valueobtained from the uncoated cemented carbide substrate.

In the following Table II, the metal cutting performance of coatedinserts prepared in accordance with this invention is shown and comparedwith the corresponding performance of uncoated inserts. Examples 11through 17 were 1/2"×1/2"×3/16" disposable cutting inserts, coated withAl₂ O₃ .[.at 1050° C..]. by the vapor deposition technique disclosedabove for Examples 1 through 10. A range of coating thicknesses of from1-10 microns was used. These inserts were then used to machine SAE 1045steel, 190 BHN hardness, at 700-1000- and 1500-surface-feet-per-minutespeeds, .010 inch per revolution feed, and .100 inch depth of cut. Thecutting times to a flank wear of .010 inch are shown in Table II, alongwith the crater wear depth at the .010 flank wear time. The transverserupture strengths are also given. For comparison purposes, the cuttingperformance and strengths of the uncoated substrate material, Examples18 and 19, a commercially available solid aluminum oxide base (89% Al₂O₃, 11% TiO) insert--Examples 20-22--and a TiC coated cemented carbideinsert--all run under the same conditions--is also shown in Table II.

                                      TABLE II                                    __________________________________________________________________________                           Coating                                                                            Cutting                                                                           Time to                                                                             Crater depth                                                                         Transverse                                              thickness                                                                          speed,                                                                            .010 flank                                                                          at .010                                                                              rupture strength                 Example                (microns)                                                                          s.f.p.m.                                                                          wear (min)                                                                          flank wear                                                                           (p.s.i.)                         __________________________________________________________________________    11   Al.sub.2 O.sub.3 coating on cemented carbide.sup.1                                              1    700 9     .003"  260,000                          12   Al.sub.2 O.sub.3 coating on cemented carbide.sup.1                                              4    700 32    .002"  250,000                          13   Al.sub.2 O.sub.3 coating on cemented carbide.sup.1                                              7    700 51    .001"  235,000                          14   Al.sub.2 O.sub.3 coating on cemented carbide.sup.1                                              10   700 51    .008"  210,000                          15   Al.sub.2 O.sub.3 coating on cemented carbide.sup.1                                              7    1,500                                                                             .sup.2 4.2                                                                          .sup.3.0003"                                                                         235,000                          16   Al.sub.2 O.sub.3 coating on cemented carbide.sup.4                                              7    1,000                                                                             17    .007"  175,000                          17   Al.sub.2 O.sub.3 coating on cemented carbide.sup.4                                              12   1,000                                                                             26    .003"  160,000                          18   Uncoated carbide.sup.1 700 4     .004"  270,000                          19   Uncoated carbide.sup.1 1,000                                                                             5     .010"  230,000                          20   Solid Al.sub.2 O.sub.1 700 51    .001"   90,000                          21   Solid Al.sub.2 O.sub.1 1,000                                                                             30    .002"   90,000                          22   Solid Al.sub.2 O.sub.1 1,500                                                                             .sup.2 4.5                                                                          .sup.3.0002"                                                                          90,000                          23   TiC coating on cemented carbide.sup.1                                                           5    700 18    .011"  175,000                          24   TiC coating on cemented carbide.sup.1                                                           5    1,000                                                                             4     .011"  175,000                          __________________________________________________________________________     .sup.1 72% WC, 8% Tic, 11.5% TaC, 8.5% Co.                                    .sup.2 To .004" wear.                                                         .sup.3 At .004" flank wear.                                                   .sup.5 71% WC, 12.5% TiC, 12% TaC, 4.5% Co.                              

It can be seen that the cutting performance of the cemented carbide toolmaterial is very substantially improved by the Al₂ O₃ coating and thatthis improvement is substantially greater than a TiC coating on the samesubstrate. It is also evident that the amount of improvement obtained isdependent upon coating thickness up to a value of about 7 microns andthat some evidence of performance decline occurs at 10 microns. At theoptimum thickness value of 7 microns for this substrate, the performanceof the Al₂ O₃ coated tool was equivalent to that of the solid Al₂ O₃tool at all three speeds tested. The strength of the Al₂ O₃ coatedinserts was, however, considerably higher than solid Al₂ O₃ and higherthan the strength of the same substrate with a TiC coating.

It should be noted that, because of strength limitations, it has notbeen feasible to use solid aluminum oxide cutting materials indisposable cutting inserts of the type used in pin-type holders. Theseinserts have a centrally disposed hole for the reception of a pin whichlocks the insert in place. The strength of such inserts must besufficient to resist the locking stresses. The strength of the presentcoated materials is sufficiently high to enable their use in suchinserts. The present invention therefore makes possible the use of aninsert, in such applications, having a higher wear resistance than anycomparable insert presently available.

The following Table III shows the performance of the coated inserts ofthe invention in cutting a high temperature nickel-base alloy,specifically Inconel 718 in the solution-aged condition (BHN 390hardness). The results, Example 25, are compared with the performance ofan uncoated cemented carbide of the same composition (Example 26), andin addition with a commercial solid aluminum oxide tool (Example 27).The inserts were of the negative-rake disposable type (indexable andinvertible) and were 1/2"×1/2"×3/16". The cemented carbide substrate forExamples 25 and 26 was 94% WC and 6% Co. The substrate was coated withAl₂ O₃ .[.at 1050° C..]. by vapor deposition process described above inconnection with Examples 1 through 10.

                  TABLE III                                                       ______________________________________                                                            Coating  Time to                                          Ex-                 thickness                                                                              .020" flank                                      ample Insert type   (microns)                                                                              wear (min.)                                                                           Comments                                 ______________________________________                                        25    Al.sub.2 O.sub.3 coating on                                                                 2.5      8.5                                                    cemented carbide                                                        26    Uncoated cemented      5.4                                                    carbide                                                                 27    Solid Al.sub.2 O.sub.3 <1      Rapid edge                                                                    breakdown.                               ______________________________________                                    

The performance of the insert coated with 2.5 microns of Al₂ O₃ wassignificantly better than that of the uncoated cemented carbide insertof the same substrate composition. From tests with other coatingthicknesses, it has been determined that the optimum thickness for thiskind of machining (i.e., high temperature alloys) is in the 1-3 micronrange. Thicknesses greater than 3 microns in these tests decreased toollife. The superior strength of the Al₂ O₃ coated tools is amplydemonstrated by the rapid failure of the solid Al₂ O₃ tool in Example27, whereas no breakage or chipping was observed in the Al₂ O₃ coatedtools, Examples 25.

The foregoing is a description of illustrative embodiments of theinvention, and it is applicant's intention in the appended claims tocover all forms which fall within the scope of the invention.

I claim: .[.1. A high-strength, high-wear-resistance coated cementedcarbide product comprising and other materials..]. .[.5. The coatedcemented carbide insert of claim 4 having a centrally disposed holetherein, the insert adapted to be mounted in a pin-type toolholder..]..[.6. The coated cemented carbide product of claim 1 in which thecoating is less than 15 microns in thickness..]. .[.7. The coatedcemented carbide product of claim 1 in which the cemented carbidesubstrate comprises titanium carbide and a matrix selected from thegroup consisting of iron, nickel and cobalt..]. .[.8. The coatedcemented carbide product of claim 7 containing tantalum carbide..]..[.9. The coated cemented carbide product of claim 1 in which thecemented carbide substrate comprises tungsten carbide, titanium carbideand tantalum carbide and a cobalt matrix..]. .Iadd.10. A cementedcarbide tool provided with a surface coating produced by coating atleast a portion of the surface of the cemented carbide with a layer 1.0to 20 microns thick of refractory aluminum oxide. .Iaddend. .Iadd.11. Acemented carbide tool as claimed in claim 10 wherein said surfacecoating has a thickness of between 1.0 and 10 microns. .Iaddend..Iadd.12. A cemented carbide tool as claimed in claim 10 wherein saidcemented carbide is composed of a carbide of a metal selected from thegroup consisting of tungsten, titanium, tantalum and niobium, or a mixedcarbide of tantalum and niobium, and a binder metal selected from thegroup consisting of cobalt, iron, and nickel. .Iaddend.